In recent years, portable electronic equipment such as notebook computers, personal digital assistants (PDAs), cellular phones and camcorders are becoming popular increasingly. Accordingly, a secondary cell used as the power supply for the portable electronic equipment is highly demanded to be small and have high capacity, high cyclic lifetime and the like. A known secondary cell satisfying such demands is, for example, a lithium-ion secondary cell using a non-aqueous electrolyte containing lithium salt.
For such a lithium-ion secondary cell, a lithium-containing transition metal composite oxide such as LiCoO2, LiNiO2, LiMn2O4 are used as a positive electrode active material. A carbon-based material is used for a negative electrode, and an electrolyte having lithium salt such as LiPF6 or LiBF4 dissolved into a nonaqueous solvent is used as a non-aqueous electrolyte.
The lithium-ion secondary cell has characteristics that its energy density is higher as compared with a conventionally used Ni—Cd cell or Ni-MH secondary cell and far superior in view of safety to a secondary cell using lithium metal. Thus, a large quantity of lithium-ion secondary cell is being used as the power supply for portable electronic equipment.
For example, a positive electrode using a positive electrode active material such as LiCoO2 or LiNiO2 is produced as follows. First, a mixture of cobalt oxide or nickel hydroxide and lithium carbonate, lithium hydroxide or the like is calcined in the air or oxygen at a temperature of about 600 to 1000° C. to produce a composite oxide. The bulk composite oxide is pulverized into sizes of several micrometers to several tens of micrometers and further classified by screening or the like, if necessary. The composite metal oxide powder obtained as described above is suspended together with a conductive agent and a binder into an appropriate solvent to prepare a slurry, which is then applied onto a collector (metal foil) and dried to form a thin plate so to produce a positive electrode (see Japanese Patent Laid-Open Applications No. 11-135119 and No. 11-149925).
But, the lithium-ion secondary cell using the above-described conventional positive electrode has a disadvantage that a faulty voltage drop tends to occur at the time of initial charging, resulting in that a production yield drops, battery performance lowers, or the like. Such phenomena were studied to find that a particulate metal impurity is often included in the positive electrode active material produced by a conventional production method, resulting in causing problems. The particulate metal impurity was mingled in a quite small amount, which was such a small amount not causing a problem even when the content of an impurity metal element in the positive electrode active material as a whole was analyzed. Therefore, such inclusion was not found by a conventional production process or analyzing method.
An object of the invention provides a positive electrode active material for a secondary cell that allows to increase a production yield of a non-aqueous electrolyte secondary cell and improves cell performance by establishing a method for analysis and evaluation of factors (such as a particulate metal impurity) that lower the cell performance and production yield. Another object of the invention is to provide a non-aqueous electrolyte secondary cell using such a positive electrode active material. Still another object of the invention is to provide a method for analysis of a positive electrode active material for a secondary cell that can analyze and evaluate factors which lower the cell performance and production yield.